Some species, most of them insects, benefit increasingly from man 's activities. Modern, large-scale agriculture offers them a paradise. Crops are grown year after year in the same or nearby fields, helping insects populations to build up. Many of the worst pests are insect invaders from foreign countries that have left their natural enemies behind and so are as free as man himself from the check of nature's balance. Agricultural scientists try hard to find ways to check insect pests by tricks of cultivation. They import the ancient enemies of invading foreign insects and foster the resident enemies of invading foreign insects.

1215. Some species that benefit from man's activities

(a) have large populations because of modern agriculture

(b) are killed by modern techniques

(c) are generally killing all the living organisms

(d) have been used as a check of nature

1216. According to the passage, the ancient enemies of invading foreign insects